1. Field of the Invention
The present invention relates to a focus detection system for a camera or the like, and more particularly to a focus detection system which detects a focus position of an object lens by the relative positional relation of a plurality of secondary object images formed by light fluxes passing through a plurality of areas in a pupil of the object lens of the camera.
2. Description of the Prior Art
A focus detection system which uses a refocusing system arranged on an image plane side of a focusing lens to detect a focus state of the focusing lens by detecting a relative positional relation of a plurality of secondary object images formed from a primary object image which is formed by the focusing lens based on light fluxes from a plurality of areas in a pupil of the focusing lens, has been proposed such as by Japanees Patent Application Laid-Open No. 95221/1977. FIG. 1 shows a prior art optical system for the focus detection systems which uses the refocusing system. A problem encountered in this system is discussed below.
Numeral 1 denotes a focusing lens which corresponds to an object lens of a camera, numeral 2 denotes a view field mask arranged on or near a predetermined focusing plane of the object lens 1, numeral 4 denotes a pupil division prism including two prisms 4a and 4b having opposite angles of inclination, numeral 4' denotes a pupil division mask, numeral 5 denotes a refocusing lens having the pupil division mask 4' as a pupil thereof, and numeral 6 denotes a field lens arranged near the predetermined focusing plane of the object lens 1 for focusing the pupil of the refocusing lens 5 near the pupil of the object lens 1. The pupil division prism 4, pupil division mask 4', refocusing lens 5 and the field lens 6 constitute a refocusing system. Numeral 7 denotes photo-electric conversion means having two photo-electric conversion device arrays 7a and 7b arranged near the image plane of the refocusing system in correspondence to the two prisms 4a and 4b.
The pupil of the object lens 1 is divided into two pupil areas 8a and 8b by the refocusing system. A light flux passed through the pupil area 8a forms a primary object image near the view field mask 2 and then forms a secondary object image having a parallax near the photo-electric conversion device array 7a by the refocusing lens 5 through the field lens 6 and the prism 4a. A light flux passed through the pupil area 8b of the object lens 1 also forms a primary object image near the view field mask 2 and then forms a secondary object image near the photo-electric conversion device array 7b by the refocusing lens 5 through the prism 4b. Since a relative position of those two secondary object images varies with a focus status of the object lens 1, the focus status of the object lens 1 can be detected by detecting the relative position of two secondary object images.
If for example, the focusing plane of the object lens 1 is on the predetermined focusing plane, the relative position of the two secondary object images respectively coincides with reference positions, but if the focusing plane of the focusing lens 1 is in front of the predetermined focusing plane, that is, in a near-focus state, the two secondary object images are shifted from the reference position in directions of arrows 9a and 9b, respectively. If the focusing plane of the object lens is behind the predetermined focusing plane, that is, in a far-focus state, the secondary object images are shifted in directions of arrows 10a and 10b, respectively.
In the focus detection system shown in FIG. 1, the pupil division prism 4 plays an important role to divide the pupil of the object lens 1 but it creates a unique distortion in the secondary object image by a prism function.
For example, when a square grid pattern 12 shown in FIG. 2 is viewed through a prism 13, an image 11' being distorted relative to an ideal image 11 shown in FIG. 3 appears. Even if aberrations of the refocusing lens 5 are compensated for, the distortion still appears in the secondary object image.
When the images of the view field mask 2 on the photo-electric conversion device arrays 7a and 7b in the focus detection system 1 of FIG. 1 are viewed from the image plane side, they appear as shown in FIG. 4. The aperture images 14a and 14b of the view field mask 2 include arcuate distortions created by the prisms 4a and 4b. Those distortions cause the reduction of focus detection accuracy. This is explained in connection with FIG. 5.
It is assumed in FIG. 5 that the object has a dark and light edge pattern and boundaries of dark and light edges are inclined with respect to the direction of the photo-electric conversion device arrays 7a and 7b. It is also assumed that the focusing lens 1 is in an in-focus state to the object and secondary object images are focused around the photo-electric conversion device arrays 7a and 7b. Numerals 15 and 16 respectively denote a light area and a dark area of the secondary object images of the edge pattern.
Since the object lens 1 is in the in-focus state, the two secondary object images are formed at essentially same positions with respect to those of the distored aperture images 14a and 14b of the view mask and the border lines of the dark and light areas of the secondary object images coincide with positions 17 and 18 at which the secondary object images cross the aperture images 14a and 14b of the view field mask.
However, the position at which the photoelectric conversion device arrays 7a and 7b cross the dark-light border lines are positions 19 and 20 for the upper image shown in FIG. 5 and positions 19' and 20' for the lower image shown in FIG. 5. Accordingly, the two secondarly object images are laterally spaced by distances d and d', respectively.
As a result, the signals from the photoelectric conversion device arrays 7a and 7b indicating that two secondary object images respectively deviated from the reference positions are produced. Consequently, an out-of-focus state is detected although the focusing lens 1 is inthe in-focus state.
Such an error is created not only when the object has an oblique edge pattern but also when the object has a dark-light distribution which is normal to direction of the photo-electric conversion device array.